Images captured using digital cameras can suffer from color artifacts such as colorful spots and lines, particularly in highlight areas and sharp brightness edges. Such color artifacts are especially problematic on small mobile cameras with small and imperfect lenses. Color artifacts are often responsible for undesirable colorful spots and lines that should not be present in a captured image, for example bright dots/lines (specular areas) and sharp brightness edges (switching area between sky and obscure objects). Color artifacts can manifest themselves as “fringes” of high-contrast edges, resulting, for example, from the failure of the camera lens to focus all colors at one point on an image sensor.
Color spot artifacts, which are one type of color artifact, are undesirable colorful spots or lines, typically occurring around bright spots and high-contrast edges in an image. Color spot artifacts can be caused by several factors. In imaging systems using under-Nyquist sampling, color artifacts such as small bright dots/thin lines can be caused by sampling at the under-Nyquist rate. Color spot artifacts can also be caused by specular reflection of light, false correction of bits per channel, non-uniform color response of sensors, and imperfect demosaicking algorithms.
Another type of color artifacts, known as chroma or color aberrations, manifest as fringes of color along boundaries that separate dark and bright parts of an image. Color aberrations can occur when an imperfect lens fails to focus different colors of light at the same convergence point. This is because lenses have a different refractive index for different wavelengths of light (known as the dispersion of the lens), and because the refractive index decreases with increasing wavelength. Since the focal length of a lens is dependent on the refractive index, different wavelengths of light can be focused on different positions of the image sensor. Color aberration can also be caused by different wavelengths of light being focused at different positions in the focal plane. As a result, noticeable colored edges can appear around objects in the image, particularly in high-contrast areas.